Is it taboo to mod a speaker?

I often read and hear about modifications done to source equipment, amplifiers, DACS, and ocassionally I read about someone doing stuff to the crossover portion of a loudspeaker. I myself plan to have someone modify my Pioneer DVD in a few months. However, I rarely read about anyone modding a loudspeaker by exchanging drivers. I have never heard of anyone exchange or upgrading to driver ABC from drive XYZ. Is it done? Does anyone do this professionally?
Modding speakers is not uncommon but completely changing drivers is. While this "might" be a step forward in some designs, doing such without looking at the design as a whole could result in a BIG step backwards. Sean
I changed my B&W800's to active bi-amping. The change and rewiring of the speakers was designed by Dan D'Agostino of Krell. I wouldn't have done it without him.

If someone REALLY knows what they are doing, it can be done, but be very careful!

I own Magnepan speakers and people mod the heck out of them! fuse bypass, tweeter attenuation bypass, crossover upgrades and internal re-wiring. I won't do any of it(affraid of lost resale value) and maggies are unusually mod friendly. I wouldn't think of trying to upgrade a speaker, if for no better reason then the lost value. I am baffled how some folks think they know more about designing speakers then those who do it for a living! Of course a speaker is designed with in a certain price range(compromises are made) but they work with what they have as a whole! taking or adding anything to that is a risk- the old the total is greater then the sum of its part theory. I wouldn't plan on getting much out of any modified component, its always so hard to prove(I recall more then 1 dispute here is due to a mod)- of course all of this doesn't mean that some folks don't get lucky either- do it right and you may never worry about resale value :o)
Often the high cost of modifying speakers...not to mention the uncertain best applied to simply upgrading to an entirely new speaker...which exhibit some of the attributes one was looking to modify in their original speakers...
Everyone else has covered pretty well. The only mods I've done and would recommend to speakers is deadening the cabinets with "dynamat" equivalent. That one is usually an improvement with no downsides/tradeoffs. Changing a driver and doing it properely is quite a bit of work and usually not worth it. I've read conflicting claims about the foam rings to help with diffraction. Someone has done some measurements on them and they don't quite work as the manufacturer claims--I believe.
Most speaker mods involve things like crossover component quality upgrades, internal wire upgrades, or binding post upgrades, in addition to various cabinet-deadening strategies such as coatings, adhesives/sealants, or sheet appliques. Some folks go as far as changing, removing, or augmenting the cabinet stuffing materials, but both that, and any driver changes done in isolation (in other words, not as a manufacturer-sourced upgrade in conjunction with adequate testing and concurrent crossover mods as needed), will stand a very good likelihood of changing the sound in unintended - and probably detrimental to accuracy - ways. The only other things commonly done involve various stands or footers. I agree with Phasecorrect: If you want an upgrade here, you're better off looking for different speakers in most cases.
Best done by the original designer of the speaker!Changeing XO's or using up to date parts is a plus.Little tweeks like changeing Binding Post and Internal Wiring can be DIY.
To address the question about changing drivers, this is not commonly done for several reasons. Crossover and bass loading are tailored to the specific drivers as a system. Each driver has its own physical and electrical charactoristics. Sometimes very similar tweeters can be switched out without changing the crossover, but otherwise its not just as easy as swapping one driver for another.
Yes,If you change Driver's you have toi redo the XO's then have the Speaker's tuned again.I have to get mine done after adding a Mid\Bass Driver,but the sound is better than having the stock driver's so I am happy with the results because they are very close to the Stock models as far as the freq.'s crossover points.
Moding speakers is always something(like anything else), that you can try, indeed. You just have to do "one thing at a time", and see what you've got when you do a modification! If it works, you leave it. If it doesn't, you sould probably change it back!
I've rewired speakers, "braced" cabinets", changed drivers, and changed "single post" speaker designs into "dual post/bi-wire/bi-amp" capable speakers...but never change crossovers. The results are sometimes improvements, but some times not!
In general, if you're not a speaker designer per se, you can only cross your fingers. But, you try and see, and then you'll know for sure!
Good luck
I have bought some audioquest cable and replaced wire.I think the designer did his or her best and is best left alone.It reminds me of wisdom . if you are looking to change something,you may well find it somewhere else and sell your speakers unmodified for a good price
I'd add, glue on felt ring around the tweeter, plug port if too much bass, in addition to Zaikesman's excellent post.
...And of course, I think we've forgotten to mention the #1 speaker mod of all time - listening with the grilles removed.
Upgrading the caps in the tweeter's filter will greatly help some speakers. Some folks think this is the most important passive component in the entire system. Change with a like value.

You can go to forums like the Madisound and it is likely that someone has experience with your speaker or one like it.

I remain,
I replaced all the resistors in in my Dunlavy SC-III's with Mills and removed the all the quick-connects on the crossover and soldered the internal wires directly to the PCB. Last week I replaced all but one of the Solen capacitors with Auricaps, the remaining cap is 62 mF and there's no Auricap equivilent (even if there was I don't think I could afford it). I also added a set of Cardas binding posts and soldered the internal biwire leads to it for single wiring.

Each mod was an improvement, but none so much as the Auricaps. These caps are fanstastic! The music is smoother, more detailed, more dynamic, more everything, etc, etc, etc...

Next I'll remove the quick-connects at the driver end and replace the inadequate wooden plinth with one made of granite. I'd like the upgrade the internal wiring but with five drivers per side and six foot tall cabinets this could get expensive. Somebody please stop me!